146 VERTEBRATA : AVES OR BIRDS. 



there is a series of muscles reaching from the pelvis to the 

 toes, and so arranged that the mere weight of the bird 

 flexes the toes, and thus enables it to sleep in perfect 

 security, even though perched on one foot. 



The nervous system of Birds is well developed, and the 

 cerebral hemispheres are superior in size to the other 

 parts ; but the cerebrum has no convolutions ; and there 

 is no corpus callosum ; and the cerebellum is transversely 

 grooved. The optic lobes, which in Mammals are small 

 and concealed by the brain, are comparatively large in 

 Birds, and plainly seen without dissection. 



Birds have the most piercing and distinct power of sight, 

 and they seem to distinguish objects near or remote with 

 equal facility. The eye is protected by lids, and besides 

 the horizontal eyelids there is one placed at the inner 

 angle which can instantly be drawn over the eye like a 

 curtain ; it is called the nictitating membrane, and per- 

 forms a most important office in protecting this delicate 

 organ. 



Birds, as a class, have no external ear ; but they have an 

 external tube, a tympanic cavity or middle ear, and an 

 internal ear. Owls, however, have an external conch. 



The organ of smell is situated in the base of the bill, 

 and the sense of smell is thought to be little developed. 



The tongue, in most species, has but little muscular sub- 

 stance, and the taste is probably not very delicate. 



The sense of touch is also feeble, and the nature of their 

 wincrs and feet is accordinglv unfavorable for the exer- 



O o / 



cise of this faculty. 



Birds lay eggs, and sit upon them to hatch them. The 

 egg in the ovary consists merely of the part we call yolk ; 

 it imbibes the external fluid called the white in the upper 

 part of the oviduct, and becomes covered with a shell at 



